Limited range wireless transmitter



April 6, 1943. H. L. KNOEBEL I LIMITEDRANGE WI RELES S TRANSMITTER IFiled Sept. 22,. 1941 Patented Apr. 6, 1943 LIMITED RANGE WIRELESSTRANSMITTER Haincs L. Knoebel, Philadelphia, -Pa., assignor to PhilcoRadio and Television Corporation, Philadelphia, Pa., a corporation ofDelaware 7 Application September 22, 1941, Serial No. 411,906

9 Claims.

This invention relates to limited-range wireless transmitters such asmay be employed in wireless record players. and more particularly theinvention relates to wireless record players in which a loop antenna isused to radiate a modulated carrier wave to an associated radioreceiver.

In the past it has beencommom practice to employ record players in whicharadfo-frequency signal is transmitted from the record player to theradio receiver over the common power line. In such arrangementchanges inimpedance of I the power line may cause changes in the modulation on thecarrier, and in many cases certain types of load on the power line willcause hum modulation on the received wave.

It has also been known in the past to use capacitive radiators fortransmitting the modulated oscillations of the record player to thereceiver, hutnone of these commonly employed systems has proven entirelysatisfactory for reasons which will be explained hereinafter.

It is a. principal object of the present invention to provide a limitedrange wireless transmitter.

Still another object of theinvention is to proventoverloading in theinput of a loop-equipped radio receiver when picking up a modulatedcarrier signal from a nearby wireless record player having a tuned loopradiator.

A further object of the invention is to provide simple but effectivemeans for changing the output of a wireless record player over a widerange.

nected in the circuit of the crystal .pickup unit I is a compensatingnetwork I which consists of the resistance 8 and the condenser 9. Thisnetwork is designed to determine the frequencycharacteristicof thecrystal pickup I. In parallel with this network are theserially-connected resistors ill and II, the junction point of which isconnected to the terminal I! of the switch l. The purpose of thisarrangement will be explained presently.

The oscillator section of the circuit includes the tapped tank coil I3together with certain of the electrodes of the multi-grid tube 6. Theseelectrodes comprise the virtual cathode I, the screen grid iii, thecontrol grid l6, and the anode H. The circuit resembles a Hartley typeoscillator in that the grid and anode electrodes l6 and I1,respectively, are connected to opposite ends of the tank coil IS (thegrid through coupling condenser i8), whilethe virtual cathode i4 isconnected to, the intermediate tap on the said tank coil. The virtualcathode M, which maybe constructed in the form of a grid, derives itssupply of electrons, of course, from the real cathode I9. The stream ofelectrons passing. from'the cathode H to the virtual cathode it ismodulated, however, by the signal on the control grid 5, and thus theoutput of the oscillator is modulated in accordance with the signal fromthe phonograph pickup device I.

In order to provide apredetermined bias between the cathode l5 and thesignal input grid 5,

' a cathode bias resistor 2i) may be connected be- Another object or theinvention is to reduce V pickup unit I which for purposes of thisdescription will be referred to as a crystal phonograph pickup, butwhich might be any sort of phono- V graph pickup or might even be amicrophone or other translat ng device, e. g., a short wave converter,This pickup is connected through termi- IlfliS- 2 and 3 of the switch 4to the control grid 5 of the oscillator-modulatortube G. Aiso'coniii)tween the cathode and ground, while the bias onthe oscillator controlgrid i6 may be determined by proper choiceoi a grid leak 2i. Platecurrent for the tube may be derived from a suitable source In accordancewith the inve ation, the tuning condenser is connectable across thewhole ora portion of the tank coil It by means of switch 28 which isganged with switch 4. The purpose of this arrangement will be describedpresently.

Coupled to the tank coil I3 by way of the seccndarycoil 21, whosemidpoint may be grounded, is the loop radiator-28. Preferably the loop28 is of the low impedance variety disclosed in the copendingapplication of Wm. H. Grimditch, Se-' rial No. 389,987, filed April 23,1941'. r In practice this loop may comprise one turn of wire hav ng agenerally rectangular configuration, and may conveniently havedimensions, for example, of. the order of 8 by 12 inches. The loop mayadas shown.

receivers equipped with vertically positioned loop antennas. In onephysical embodiment of the invention, the loop was placed in the insideof the rear of the cabinet of the record player, and

v by the ordinary capacitance antenna varies inversely as the distance,whereas, the magnetic induction field strength set up by a loop, such asI the one under consideration, varies inversely, as

the cube of the distance from the loop and becomes virtuallynon-existent shortly beyond the useful range of the device. Thus, it isapparent that a relatively strong field may be used in the desiredworking range without creating an extensive interfering field.

vantageously, be disposed in a vertical plane to better enable receptionof the radiated signals by the amplitude of this wave bear the. desiredratio to that of the fundamental. As will be explained in detailhereinafter, the operating'frequency of the oscillator may be altered bychanging the resonant frequency of the tank circuit. a

It has been found, however, that when the percentage of modulati6rl isadjusted to afiord optimum reception of the fundamental wave, the secondharmonic wave will be seriously overmodulated, and considerabledistortion will re sult unless means are provided for substantially Inthe operation of the device in accordance with the invention, if therecord player is some distance from the radio receiver, e. g., more than4 or 5 feet, the gang switches '4 and 26 are set Under these conditionsthe entire voltage'developed across the pickup I is applied to the grid5. Likewise, the tank tuning condenser is connected through the switch26 across the lower half of the tank coil I3. This condenser ispreferably adjusted to tune'the plate circuit to a suitable frequency inor near the broadcast The voltage ap-- band, say from 540v to 5'70 kc.plied on the control grid 5 will then modulate the oscillations whichare occurring in the oscillatory circuit of thetube and will result in amodulated volta e wave appearing in the coil l3. This modulatedoscillatory wave will be transferred to the coil 21 andradiated from theloop antenna 28, and may be picked up within the service range of thedevice by a radio receiver tuned to the oscillator frequency. Ifit isdesired to operate the record player a very short distance, e. g., lessthan 4 or 5 feet,

from the radio receiver, it may be found that reducing the degree ofmodulation under these conditions. In further accordance with thepresent invention, means are provided for reducing the magnitude of themodulating potential during second-harmonic operation of the wirelessrecord player. Experience has shown that the modulating signal is bestreduced in magnitude by or more during second harmonic operation, and inone satisfactory embodiment of the device it was found desirable toreduce the modulating signal to 35% of that employed during normal orfundamental operation. This reduction is accomplished in the recordplayer constructed in accordance withthe invention by adjusting the gangswitches 4 and 26 to. their second position. The crystal pickup isvnowconnected to the grid 5 of the tube through the voltage divider circuitconsisting of the resistors 10 and II, and the voltage applied to thegrid 5 is reduced in the ratio of 'The tuning condenser 25 is nowconnected across the whole tank coil l3 rather than across only aportion of it, as in normal operation. If the total inductance of thetank coil I3 is properly related to the inductance of the anode portion'thereof, it willbe possible approximately to halve "rived at by trial,but ingeneral, where a single the strength of the signal radiated willbe such as to cause an excessively high voltage, for example, in thenature of one volt or even more, to appear on the input of the firsttube of the receiver. This may result in an abnormally strong AVC actionin the receiver with resulting distortion,

or may even cause the receiver to go into R. F.

It is, therefore, desirable to reduce the output of the record playerunder such conditions by a oscillation with blocking of the incomingsignal.

wide margin, for example, by 9. 100-1 ratio. However, changing theoutput of an oscillator circuit over so wide a range is not easilyaccomplished, since the stability and other desired characteristics ofthe oscillator depend on maintaining a predetermined optimum outputlevel. According to the present invention, the output level of theoscillator may be maintained substantially con stant, while thefrequency is reduced by one half,

so that the wave picked up by thev receiver under these conditions'isthe second harmonic of the oscillator wave, rather than the fundamental.

Since the power contained in a harmonic is: normally small, it mayeasily be arranged to have tuning condenser is employed, as shown in thedrawing, the inductance in the resonant circuit at the lower frequencyshould be approximately four times the inductance in the resonantcircuit at the higher frequency. The record player will now operate inthe same manner as previously described excepting that the voltageapplied to the grid 5 will be materially lower and the frequency of theoscillator will be approximately one-halfof its normal frequency, andthe radio receiver, instead of receiving the fundamentalof theoscillator frequency, will receive the second harmonic. Thissecondharmonic may be weaker than the fundamental in the order of about 1-100.and the voltage imposed on the receiver will,

therefore, be well within the limits required for satisfactoryreception.

In order to permit the changing from the local" position to the distanceposition of the,

quency as the fundamental in the secondcase.

While the above-described embodiment ofthe invention uses one-halffrequency and the second harmonic, it is contemplated that one-thirdWhile the invention has been illustrated and described in its specificapplication to a record 1. A wireless record player or the like,comprising a source of current of modulating .frequency, an oscillator,a modulator, and a radiator, means for changing the frequency of theoscillator so that the frequency which is to be picked up is either thefundamental or the second 1 harmonic of the oscillator frequency, andmeans for simultaneously changing the modulation so that the percentagemodulation is reduced when the frequency which is to be picked up is thesecond harmonic of the oscillator frequency.

'2. A wireless record player as defined in claim 1, in which theradiator is an inductive loop.

3. A wireless record player or the like, comprising an oscillator, asource of modulating current, means for modulating the oscillator withsaid current for transmitting a modulated wave to a radio receiver at aredetermined frequency,

said predetermined frequencybeing optionally either the oscillatorfrequency or twice the oscillator frequency, and switch means to reducethe A 6A7 tube,

fundamental or a harmonic of the generated wave frequency, and means forsimultaneously known separate or combined oscillators and mod- I ulatorsmay be employed. v

changing the degree of modulation of the generated carrier wave wheneverits frequency is changed by said last-named means.

7. A wireless transmitter for transmitting a desired signal to a radioreceiver from different positions within a short distance range of thereceiver, comprising means for generating a carrier wave ofpredetermined frequency suitable for operation of the receiver over thegreater dis-' tances in said range with the receiver tuned to saidfrequency, manually operable means for changing thefrequency of thegenerated carrier wave to a predetermined fraction of said firstnamedfrequency to operate the receiver-over very short distances withoutchanging the tuning of the receiver, thus causing the receiver to re-"ceive a harmonic of the fractional frequency,

whereby the transmitter is operated at substantially constant output butthe signal received at the receiver is substantially varied in strength,and means for modulating the carrier. wave with the desired signalduring either of the aforesaid transmitting operations.

8. A wireless transmitterfor transmitting a desired signal to aloop-equipped radio receiver from different positions within a shortdistance range of the receiver, comprising means for generating acarrier wave of predetermined fre-. quency suitable for operation of thereceiver over the greater distances in said range with the receivertuned to said frequency, manually operable means for changing thefrequency of the generated carrier wave to a predetermined fraction of:said first-named frequency to operate the receiver over very shortdistances without chang- 1 ing the tuning of the receiver, thus causingthe receiver to receive a harmonic of the fractional degree ofmodulation when the predetermined frequency is twice the oscillatorfrequency.

4. In a wireless transmitter for transmittin a desired signal to a radioreceiver within a short distance range, means for generating a carrierwave, means for modulating said carrier wave with said desired signal,means operable at will to change the frequency of said carrier wave inthe ratio of n to 1, where n is an integer greater than 1, and means forsimultaneously varying the degree of modulation of the carrier wavewhenever its frequency is changed by said last-named means. i p

5. A wireless record player, comprising means for generating a carrierwave, a signal pickup device, means for modulating said; carrier wavewith the signal from said device, means operable at will to change thefrequency of said carrier wave in the ratio of n to 1, wherent is an in-.teger greater than '1, and means for simultaneously varying the'degreeof modulation of the carrier wave whenever its frequency is changedfrequency of the generated wave so that the frequency which is to bepicked up is either the frequency, whereby the transmitter is operatedat substantially constant output but the signal received at the receiveris substantiallyvaried in strength, means for modulating the carrierwave with the desired signal during either of the aforesaid transmittingoperations, and a loop radiator for effecting transmission to thereceiver loop through magnetic induction.

9. A wireless transmitter for transmitting a desired" signal to a. radioreceiver from different positions within a short distance range of thereceiver, comprising means for generating a car-,

rier wave of predetermined frequency suitable for operation of thereceiver over the greater distances in said range with the receivertuned to said frequency, manually operablemeans for changing thefrequency of the generated carrier wave to a predetermined fraction ofsaid firstnamed frequency to operate the receiver over very. shortdistances without changing the tuning of the receiver, thus causing thereceiver to receive a harmonic of the fractional frequency,

whereby the transmitter is operated at substandesired signal duringeither of the aforesaid transmitting operations, and means for varyingthe degree of'modulation of the carrier wave hl1mu1t:ne0usly' with, thechange in frequency ereo HAINES L, KN OEBEI

